Power conversion devices known in the art such as resonant DC-to-DC power converters are used to provide power in audio applications and in a number of other electronic applications. A popular type of resonant power converters is the well known class D power converter.
Class D converters operate by alternatively turning on and off a pair of power MOSFETs (metal oxide semiconductor field effect transistors) in response to a square wave switching signal. Current from the MOSFETs passes through a resonant capacitor and primary windings of a high frequency transformer which efficiently converts the pulsed high voltage MOSFET output to a suitably lower output voltage. Output from the secondary coils of the high frequency transformer are rectified and filtered to provide smooth DC voltage of an appropriate magnitude for many electronic applications.
Resonant power converters have good thermal efficiency under ideal conditions because the switching signal to the MOSFETs is timed to turn each MOSFET off when the current through it is zero and the switching signal to the MOSFETs is timed to turn each MOSFET on when the voltage across it is zero. Under ideal conditions, each MOSFET is switched at a 50% duty cycle. However, in non-ideal conditions, the switching duty cycle is typically varied to provide voltage regulation as the input voltage varies. Variation in the duty cycle causes at least one of the MOSFETs to turn off when it carries non-zero current or turn on with non-zero voltage thereby introducing thermal losses.
U.S. Pat. No. 5,986,895 to Stewart et al. discloses a method of maintaining zero current switching of MOSFETS under non-ideal conditions in a resonant power converter. This method senses the transformer primary current and the output voltage and uses digital logic to modify the MOSFET switching signal. The digital logic is designed so that the modified switching signal switches the MOSFETs at zero current even under non-ideal conditions such as asymmetric duty cycles.
Digital “brute force” methods such as the method disclosed in the Stewart et al. Patent, require a number of design compromises including increased real estate consumption on a board or chip. Also, significant additional design time and complexity is added to component layout in order to place such a digital system in close proximity to the high frequency switching components in a resonant power converter. Each of these disadvantages adds significant cost to any power converter using this technique which can make their use prohibitively expensive for many applications.